1. Field of the Invention
The present invention relates generally to data transfer and, more particularly, to systems and methods for transitioning between fragmentation modes.
2. Description of Related Art
Conventional cable modem systems include a cable modem termination system connected to one or more cable modems via a coaxial cable. The cable modem termination system communicates with the cable modems via a radio frequency (RF) interface and provides packet forwarding functions via a network side interface. Separate frequency bands are used for upstream and downstream communication on the coaxial cable. For example, the upstream communication may take place on communication channels within the 5-42 MHz frequency band and the downstream communication may take place on communication channels within the 88-860 MHz frequency band.
The upstream communication path (i.e., from the cable modems to the cable modem termination system) uses frequency time division multiple access (TDMA) communication. The cable modems transmit variable length packets (e.g., 64 to 1,518 byte packets) on one or more service flows that have associated levels of quality of service.
When a cable modem has data to transmit on an upstream communication channel, it sends a request packet in a certain time slot to the cable modem termination system requesting a specific amount of bandwidth or number of time slots in order to transmit the data. The cable modem termination system then schedules certain time slots in which the cable modem can transmit its data. For example, the cable modem termination system sends a message to the cable modem informing the cable modem that it has been granted a time slot beginning at some point in time and continuing for so many time slots. When the cable modem termination system allocates a transmission opportunity to the cable modem which is less than the cable modem asked for, the cable modem can only send a portion of its packet in the time slot(s). The cable modem sends the rest of the packet in one or more later time slots. This is called “fragmentation.”
Data-Over-Cable Service Interface Specification (DOCSIS) is the standard for cable modem systems throughout the world. DOCSIS defines fragmentation for upstream transmissions from the cable modems. The cable modem termination system initiates fragmentation whenever the cable modem termination system grants fewer time slots than what the cable modem requested. The purpose of fragmentation is to help ensure that best-effort and unsolicited grant service flows can co-exist effectively on the same communication channel.
DOCSIS defines two modes of fragmentation: (1) piggyback mode; and (2) multiple grant mode. In the piggyback mode, a cable modem requests N time slots for transmission of a packet of data and the cable modem termination system grants some number of time slots less than the N time slots requested by the cable modem. The cable modem determines how many more time slots it needs to send the rest of the packet. The cable modem then makes another request for the rest of the time slots it needs. The cable modem makes this request within the header of the first fragment it sends during the allocated time slots, thus piggybacking the request on the transmission of the first fragment. In response to the request, the cable modem termination system may grant enough time slots for all, or less than all, of the remaining packet data.
In the multiple grant mode, a cable modem requests N time slots for transmission of a packet of data. In this case, the cable modem termination system realizes that it cannot grant N time slots to the cable modem. The cable modem termination system then grants some number of time slots less than N and keeps track of how many more additional time slots are needed by the cable modem. The cable modem termination system then grants these additional time slots, thus providing multiple grants to the cable modem. The cable modem termination system may make these multiple grants via a single message called a MAP message or in multiple MAP messages.
Each of these modes has its advantages and disadvantages. For example, in the piggyback mode, the cable modem termination system does not need to keep track of how much data a cable modem has left to send (or how many time slots the cable modem still needs to send this data), thereby off-loading some of the processing from the cable modem termination system. The cable modem termination system does this at the cost of latency. It may take many cycles of the cable modem sending requests and fragments and the cable modem termination system granting time slots until the cable modem can transmit the entire packet. As a result, the overall transmission time for a packet may be many times greater than sending the packet without fragmentation.
In the multiple grant mode, the cable modem termination system keeps track of how much data a cable modem has left to send (or how many time slots the cable modem still needs to send this data). This increases the processing required by the cable modem termination system. Latency, however, is greatly reduced. The cable modem termination system may transmit multiple grants to the cable modem in the same message.
DOCSIS allows the cable modem termination system to support either fragmentation mode. All compliant cable modems are able to support either mode. In practice, however, only one of the fragmentation modes is typically used.
Because each of the fragmentation modes has certain advantages and disadvantages, a need exists for systems and methods that minimize the disadvantages and maximize the advantages of these modes.